Step 1) Make sure you’re workspace units are millimeters

Step 2) Create a 3 point arc. Dimensioned to a radiu of 30, then add a vertical line:

Step 3) Dimension the vertical line:

Step 4) Use the spline tool to create this :

Step 5)  Add 2 circles and dimension both of them to 12 (smaller ) and 50 (outer) as shown below:

Step 6)